Applications of the MAX11905 ADC for the Ultrasound Imaging System

要約

This application note discusses the application of MAX11905 ADC for ultrasound imaging systems. It also discusses the required performance of the ADC to provide optimum results for speed measurement of blood in the human body.

Introduction

In recent years, the performance of the ultrasound receivers has improved significantly. These advance technologies are made possible thanks to the high signal-to-noise ratio (SNR) and low power ADC technology that has allowed the users to utilize the high resolution 20-bit or higher ADCs like the MAX11905. In addition, low input voltage noise density amplifiers are readily available to drive these ADCs and pair with them to provide the optimum performance. When these high SNR ADCs are coupled with the low noise driver amplifiers, the total SNR and the total harmonic distortion (THD) improve tremendously. The new high SNR and low THD ADCs have significantly enhanced the CWD performance in the medical ultrasound systems. This application note provides an overview of how ultrasound is used to measure the blood flow and discusses the needed performance of the ADCs used in the CWD ultrasound system applications to achieve the desired imaging solutions.

Overview

Figure 1 shows the typical block diagram of an ultrasound imaging system. The system typically consists of transducers, high-voltage multiplexing, high-voltage transmitters, image-path receivers, digital beamformers, beamformed digital-signal processing, and display processing. Phased-array ultrasound systems can generate images of the internal organs and structures, measure the blood flow in the CWD section, and provide highly accurate blood velocity information by transmitting the acoustic energy into the body and receiving and processing the returning reflections. Definitely, the ultrasound imaging is one of the most frequently performed and reliable medical imaging procedures including the measurement of the blood flow using CWD which continuously analyzes the ultrasound waves emitted from the transducer and reflected back. The blood velocity can then be calculated based on the Dropper effect similar to the technology utilized in a radar speed detector and used by the peace officers to determine how fast a vehicle is moving.

Typical Block Diagram of an Ultrasound Imaging System

Figure 1. Typical Block Diagram of an Ultrasound Imaging System.

Figure 2 shows the Pocket Doppler BV-520T Bidirectional handheld blood flow velocity detector. For more information, refer to the Handheld Blood Flow Rate Detector.

Pocket Doppler BV-520T Bidirectional Handheld Blood Flow Rate Detector

Figure 2. Pocket Doppler BV-520T Bidirectional Handheld Blood Flow Rate Detector.

Doppler Effect

According to the well-known Doppler effect, there is an apparent difference between the frequency at which sound or light waves propagate from a source and that at which they reach an observer, produced by the relative motion of the observer and the wave source. When an ultrasound source is stationary, the detected frequency is the same as the transmitted frequency. However, if the source of the wave is in motion, there is a change in the observed frequency. If the observer is moving toward a static source, an increase in frequency is detected. Similarly, if the observer is moving away from a static source, a decrease in frequency is seen. The velocity can then be calculated based on the change of this frequency as shown in the Equation 1, which is implemented in a speed detector widely used by the peace officers to check the speed of a moving vehicle. For more information, refer to the Doppler Effect.

VH= fD × C / (2 × fT)

Where,
VH: Vehicle speed
fD: Doppler shift frequency
C: Speed of light
fT: Transmit frequency

The same technology based on this famous Doppler effect is also utilized in the ultrasound medical applications such as CWD, which is a method available in most cardiac and general-purpose ultrasound imaging systems used to precisely measure the velocity of blood flows typically found in the heart. The Doppler effect is a very sensitive and accurate detector of the motion. Therefore, it is not only used to study the blood flow, but it can also be employed to measure the tissue motion. The CWD operates on the principal of constant transmission and reception of the ultrasound waves, which can be achieved by the two transducer elements dedicated to send and receive the ultrasound signal as shown in the Figure 1. In addition, Doppler images are also produced in real-time, which makes them very useful for the ultrasound applications.

For the medical applications, the source of the transmitter is ultrasound instead of the light as shown in the Equation 2.

VB = fD × Vs / (2 × fT)

Where,
VB: Blood flow velocity
fD: Doppler shift frequency
VS: speed of ultrasound
fT: Transmit frequency

VS = 1540m/s and fT = 2MHz to 18MHz are typically known values. Therefore, the blood flow velocity VB can be determined once the Doppler shift frequency is obtained. The MAX11905 is specifically designed to measure this shift in the frequency.

For example, if VS=1540m/s, fT=2MHz, and fD=500Hz (measured by MAX11905 ADC), then the blood flow velocity is determined as follows:

VB = fD × VS / (2 × fT)
VB = 500 × 1540 / (2 × 2 × 106)
VB = 19.25cm/s

Precision Frequency Measurement Using the MAX11905

The CWD receivers are typically implemented in one of the two ways as shown in the Figure 1. In one method, the high-performance ultrasound systems typically extract a received CWD signal at the low-noise amplifier (LNA) output. The complex mixers at a local oscillator (LO) frequency equal to the transmit frequency are then used to beamform the signals and mix them to the baseband for the processing. The phase of the I/Q LOs can be adjusted on a channel-by-channel basis to shift the phase of the received CWD signal. The output signal of these mixers is then summed, band pass filtered, and converted by an ADC. The resulting baseband beamformed signal frequency range is from 100Hz to 50kHz typically. Hence, ADCs designed with this input frequency range such as the MAX11905 which can accept input frequency higher than 100KHz are used to digitize the I and Q CWD signals. These ADCs require the following characteristics for the optimal performance:

  1. Precision Measurement: To measure the frequency precisely, ADCs with 16-bit resolution or higher and sampling rate of at least 10x that of the maximum signal frequency such as the MAX11905 20-bit, 1.6Msps ADC are needed. In addition, to provide optimum input signal conditioning, high-speed and low-noise amplifiers such as the MAX4430 are used to drive the ADC.
  2. Low Power Consumption: The ultrasound CWD receivers are portable as shown in the Figure 2. Furthermore, a typical system has as many as a couple hundred of transmitters and receivers. So, the low power consumption is very essential.
  3. High Dynamic Range: The ADC needs significant dynamic range to handle both the large Doppler signals from moving tissue and the smaller signals from blood. In addition, the harmonic signals, especially the second harmonic one, generated by the nonlinear processes in the human body are strong. Therefore, it is important that the ADCs should be able to suppress those harmonics.
  4. High SNR: The ADCs require high SNR to balance the high signal attenuation in the human body.
  5. Small Size: The ultrasound CWD receivers are portable. Therefore, size does really matter.

The MAX11905 is specifically designed to provide these five needed requirements, which make it a desirable ADC for the reliable and precise ultrasound applications.

The MAX11905 is a 20-bit, 1.6Msps, single-channel, fully differential successive-approximation register (SAR) ADC with internal reference buffers. The device is designed with ultra-low power consumption that directly scales with sampling rates with only 9mW at 1.6Msps as shown in the Figure 3. Furthermore, this ADC provides excellent static and dynamic performance with high SNR approximately 98.4dB and low THD approximately -123dB as shown in the Figure 4. In addition, the MAX11905 is available in a small 4mm x 4mm package.

The MAX11905 Current Consumption vs. Sampling Rate

Figure 3. The MAX11905 Current Consumption vs. Sampling Rate.

The MAX11905 THD Performance care applications

Figure 4. The MAX11905 THD Performance care applications.

Conclusion

To measure the blood flow in a medical ultrasound system accurately, a precision ADC such as the MAX11905 is needed. The device is specifically designed to capture and measure the frequency signal from CWD receiver precisely as it possesses 20 bits of resolution and high sampling rate of 1.6Msps. Furthermore, the MAX11905 ADC which accepts input signal frequency greater than 100KHz can provide excellent static and dynamic performance and is designed to accept fully differential input signals to deliver the needed high SNR. Moreover, the MAX11905 has extremely low power consumption and small package, which are vital for the portable medical systems.

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